Peter Wilkinson Engelhard Limited, Cinderford, Gloucestershire, United Kingdom
For effective operation, gold plating baths must require the mini- mum of attention in respect of replenishment and replacement, and must produce as little sub-standard or reject work as possible. To achieve this, an understanding of the processes involved and the factors affecting them is advantageous. This article attempts to provide such an understanding in a simple manner. Special atten- tion is given to the plating of gold from acid baths containing monovalent gold complexes.
The use of gold as a contact material on connectors, Other gold(I) complexes, such as those with thiomalate and switches and printed circuit boards (PCB's) is now well estab- thiosulphate have been considered, but it is doubtful if they lished. There is also an awareness that only gold deposits from will ever find commercial application. acid (pH 3.5-5.0) gold(I) cyanide baths containing cobalt, In terms of electricity consumed, gold(I) baths are more nickel or iron as brightening agents have the appropriate efficient than gold(III) baths. Thus, assuming 100 per cent properties for a contact material. Gold deposits from: pure efficiency, 100 amp-min will deposit 12.25 g of gold from a gold(I) cyanide baths; from such baths containing organic monovalent gold complex, but only 4.07 g from a trivalent gold materials as brightening agents; or from electrolytes containing complex. gold in the form of the gold(I) sulphite complex, do not have Nevertheless, deposition from acid gold(III) cyanide baths the sliding wear resistance which is necessary for make and has merit in the plating of acid resistant substrates such as stain- break connections (1). less steels, to which electrodeposited gold normally adheres It was known for many years that deposits from acid gold(I) badly because of their passivated surfaces. Hence, these baths cyanide baths brightened with cobalt, nickel or iron contained can also be used to produce decorative gold alloy coatings on about 0.25 per cent of base metal, but this did not explain why such substrates. their densities (16.0-17.0 g/cm 3) were so much lower than the density of pure gold deposits (19.3 g/cm 3). The classical exper- iment of Munier (2), in which she dissolved the gold of the The Deposition Process brightened deposits in aqua regia vapour to reveal a polymer Figure 1 is a schematic diagram of what is thought to happen network, showed that deposits of this type were much more during the deposition of a metal from a solution of one of its complex than was formerly thought. complexes. In what follows, an earlier account (3) of the factors in- The cathode attracts predominantly positive ions to a region volved in the formulation and operation of acid gold plating near its surface which is known as the Helmholtz double layer. baths is brought up to date. Brief accounts are also included of In addition, negatively charged complex ions, such as the the electrodeposition of gold from gold(III) cyanide baths and Au(CN)--, ions present in the gold(I) cyanide plating baths, from gold(I) sulphite baths. which approach this layer become polarised in the electric field of the cathode. The Formulation of Gold Electrolytes The distribution of the ligands around the metal is thereby Range of Electrolytes Available distorted and the diffusion of the complex ion into the Helm- There are only three complexes of gold which are suitable holtz layer is assisted. for commercial use in gold plating baths. By far the most im- Finally, within the Helmholtz layer the complex breaks up. portant are the cyanide and sulphite complexes of gold(I), of Its component ligand ions or molecules are freed and the metal which the potassium derivatives are KAu(CN), and IC 3Au(S0 3), released in the form of the positively charged metal cation respectively. which is deposited as the metal atom on the cathode.
Gold Bull., 1986, 19, (3) 75 It will be appreciated from this simple picture of events at the cathode, that the ease of gold deposition from gold(I) cyanide baths, which contain gold in the form of Au(CN)2 ions, will depend on the ease with which these ions dissociate into Au + and CN ions, thus:
Au(CN) Au+ + 2C1\1- 1
The mass action law dictates that for a reversible reaction: IStOJIK? [Au(CN) 2] = a constant 2 ] [Au+ ] [C1N- 2 ayer [Bulk * where [Au(CN)2], [Aul and [CN -] are the concentrations of the respective ions in solution. ELECTROLYTE
This constant (4) provides a measure of the stability of the ess 2 Au(CN)2 complex and is referred to as its stability constant j3 •,•4gg404100 .ti5co4, (the subscript 2 indicates that the reaction is two stage). wslo?;:ig
The value of 1 2 for the Au(CN)2 ion has been determined as 10383 . This is exceedingly large and implies that the equilibrium in reaction (1) is far to the left and that the Au(CN); ion is a very stable one. Moreover, it indicates that the concentration whereas in alkaline baths it is decreased. Despite the fact that + + ions is given by equation: [Au ] of Au the stability constant of the Au(CN)2 ion is unchanged, this complex ion therefore appears to be less stable in acid than in [Au(CN);] [Au+ 1 10-383 alkaline media. The range of usefulness of gold(I) cyanide ] [CN 2 baths in acid media is limited, however, by the tendency of the Au(CN)2 complex to decompose at pH's below about 3 with This is an exceedingly low concentration and it would appear precipitation of AuCN thus: as if reasonable rates of deposition of gold from gold(I) cyanide solutions are possible only because of the polarisation Au(CN)z —> AuCN + CN- of Au(CN); ions which approach the cathode surfaces and their decomposition in accord with equation./ within the Helmholtz Both polarographic and potentiometric studies have shown layer. that the gold(III) cyanide complex Au(CN) -4 is formed in acid The electrodeposition of gold from its other complexes is gold(I) cyanide plating baths during operation, presumably by thought to occur in a similar manner. oxidation at the anode (5,6). It is important to realise that any gold oxidised to the trivalent state in this manner is not avail- able for deposition at the cathode because gold(I) and hydro- Gold(I) Cyanide Baths gen are preferentially deposited. The efficiency of the bath These may be operated under alkaline, neutral or acid con- therefore falls, not because some gold is being deposited from ditions. Under acid conditions, the value of [Au + ]is increased as gold(III) cyanide, but because the effective concentration of a result of the equilibrium reaction: gold(I) cyanide in the bath has been reduced. Also gold(III) is just as susceptible to 'drag out' as the gold(I) and the metal's H+ + CN HCN valency does not affect its price. Modern acid gold(I) cyanide baths are therefore formulated in which the formation of undissociated HCN is strongly with the addition of a mild reducing agent so as to minimise the favoured. Acid gold(I) plating baths are therefore characterised formation of gold(III) cyanide, but baths formulated several by low free cyanide ion concentrations. However, under alka- years ago can give rise to gold(III) cyanide formation especially line conditions and especially if potassium cyanide is present, under unusual plating conditions, such as wire plating. Since the value of [CN1 is greatly increased. Following equations 1 to the gold(III) complex is also very stable, it is better to prevent 3 this means that in acid baths the value of [Au '] is increased, its formation than to reduce it back to gold(I) cyanide.
76 GoldBult, 1986, 19, (3)
Gold(III) Cyanide Baths It must be stressed, however, that these baths are not used As has been indicated above, acid gold(I) cyanide plating commercially to any great extent. The deposits they yield can- baths are normally operated at pH values between 3.5 and 5. not be used in contact applications in electronics because their They cannot be operated at pH values less than 3.5, since under wear resistance is inadequate against existing gold plated con- such acid conditions the gold is precipitated as AuCN. More- tacts. Also the rates of deposition from them are slow so they over, deposits from acid gold(I) cyanide baths have little duc- cannot be used in modern high speed equipment. Moreover, tility so that plated articles cannot be subsequently deformed they are corrosive. without risk of cracks developing in the surface coating. These Apart from the chloride-free baths discussed above, other and other limitations of acid gold(I) cyanide baths have stimu- modern gold(III) baths are available commercially which do lated studies in recent years of the deposition of gold from contain chloride but which contain substances which prevent gold(III) cyanide baths. formation of chlorine at the anode. Thus, a method for the preparation of HAu(CN) 4 and the for- At present gold(III)/cobalt and gold(III)/nickel baths are mulation of gold plating baths based upon this gold complex used for plating stainless steel or chromium with flash coatings were described in 1971 (7). The baths were found to have ex- of gold (about 0.1 ,um thick). cellent plating characteristics at pH values from 0.1 to 5.0, and A gold(III)/indium bath has also been developed which gave optimum performance in the pH range 1.0 to 3.0. They yields deposits suitable for decorative applications on such sub- typically contained the gold complex, a citrate, phosphate or strates where an extremely light yellow colour is desired. tartrate, a weak organic acid and a mineral acid such as phos- phoric acid. Then in 1979 the plating of gold strikes on stainless steel Sulphite Baths substrates using gold(III) cyanide baths was claimed (8). Dissociation of the gold(I) sulphite complex occurs by the Potassium nitrate was used as the electrolyte in these baths and reaction: ethylenediamine hydrochloride as a complexant. Nickel, cobalt, copper, tin and indium could be incorporated as alloy- Au(S03)2- Au+ + 2S023 - ing constituents and the baths operated at pH's preferably less than 1.5. It is much less stable than the gold(I) cyanide complex and its The gold(III) cyanide complex was generated directly by stability constant is of the order of 10 10 or about 1028 times less warming a solution containing KAuC1 4, KNO3 and KCN, the pH than that of the Au (CN) -2 complex: of which was subsequently adjusted by additions of hydro-
[Au(S03)32 - ] chloric acid. This gave rise to evolution of chlorine at the anode 1010 and severe corrosion problems, so that significant exploitation [A1_1 + ][80,12 2 of the patent did not follow, though the deposition of gold-tin alloy coatings from baths of this type has been claimed (9). More recently (10-11), a study of the galvanic deposition of gold and gold alloys from baths based upon potassium [Au(S03)3,] gold(III) cyanide has resulted in the formulation of chloride- and [Au + I -- . 10 -1° free baths which are finding limited application for special pur- [S032 1 poses. Not only are the deposits bright, hard and relatively free from non-metallic components, but they are also ductile and relatively pore-free. They have low contact resistance which is However, at neutral and acid pH values the complex is not affected by heat. unstable. Significant reactions which occur under these con- These properties can be modified by additions of alloying ditions are: metals such as Co, Ni, In, Sn, Zn or al. Other components of the baths may include an amine, amino acid or phosphoric S03,- + F1 + —> HSO; acid, with the p11 being adjusted preferably to between 0.6 and 2.0 by the addition of phosphoric acid or sulphuric acid or mix- 2HSO; + 4e -4 S 2042- + 20H - (at the cathode) tures of these, plus citric acid, phosphate and sulphate. Ad- dition of chlorides to the baths was scrupulously avoided. 2Au(S0 3),3 - + 201-1 - + 2HSO; + 4503 + 2Au Other gold(III) cyanide baths (12,14) have also been devel- oped recently. SO: - + 2H + -4 SO2 + H2O
Gold Bull., 1986, 19, (3) 77 The formation of the dithionate ion 8,04, which is a power- 0 Co-deposited transitional metals such as cobalt, nickel ful reducing agent, leads to a gradual reduction of the gold and iron. For coatings used as contact materials on complex to metallic gold (14). Below pH 4.7 the sulphite ion connectors and PCB's, these are the most important breaks down. brighteners. Reduction of the gold complex between pH 4.7 and 8.0 can be decrased considerably by strengthening the sulphite com- In describing the brightening action of transitional metals, plex with ethylenediamine (16) or by adding an oxidising cobalt can be taken as the example but the same arguments agent, like picric acid, that will react with the dithionate at a apply to some other metals. It has been known for some time faster rate than the gold sulphite complex. However, the use of that cobalt brightened gold deposits contain carbon (18,) in fact these additives makes the bath control difficult and is not com- if a deposit does not contain at least 0.1 per cent carbon it will pletely effective. not be bright. C" labelling techniques have shown that this carbon originates in the carbon of the cyanide ligand (19). In some way the presence of cobalt is responsible for the creation of Munier's polymeric material in the gold deposit (2). Brighteners Analysis has shown that as well as carbon these deposits also Although brightness in itself is not important in gold plating contain potassium, nitrogen and oxygen. It has been suggested for electronic applications, it is associated with other desirable (20) that a complex such as KCo(Au(CN),), is formed and it is qualities such as wear resistance and lack of porosity. this material that acts as a brightener. In a plating bath this complex is visualised as being formed Brighteners for gold electrodeposits are of three types: within the Helmholtz double layer. Under the usual operating conditions it is only very slightly soluble, so that as it is formed D Organic substances such as polyethyleneimines and high it is adsorbed onto the cathode in an analogous manner to molecular weight polyamines. These operate by selective organic brighteners. Any parameter that increases the solubility absorption within the Helmholtz double layer, usually of KCo(Au(CN)2 )3, such as high pH or high temperature, would causing an increase in polarisation. They are adsorbed be expected to reduce its brightening effect. This is in accord onto the more prominent growth points so that the gold with the fact that these transitional metals act as brighteners is deposited preferentially on other parts of the cathode only in acid gold cyanide baths, whereas the semi-metals can surface, producing a levelling and brightening effect. In- brighten deposits from a much wider range of gold electro- variably the organic additive is incorporated into the lytes. If this understanding is correct we should expect to find structure of the deposit with adverse effects on its physi- K, Co, C and N present in the brightened deposits in the same cal properties, especially wear resistance. The wear resist- atomic proportions as those in which they are present in ance can, however, be improved by co-depositing a KCo(AU(CN),),, i.e. the atomic ratios of K:Co:C:N in acid gold second metal such as cadmium. deposits should be 1:1:6:6. In practice these ratios can vary enormously, the K:Co vary- 0 Co-deposited metals and semi-metals such as arsenic, ing from 1.0:0.4 to 1.0:5.5 depending on the electrolyte and the thallium, selenium, lead etc. The use of these causes de- operating conditions (21). The Co:C ratio can vary from 1:3 to polarisation and they produce only lustrous deposits 1:10 (22). However, the atomic ratio of C:K is fairly constant at from cyanide baths but fully bright deposits from sulphite 3:1 (22), and the potassium in these deposits increases linearly baths. They operate at extremely low concentrations of a with the electrolyte potassium concentration. few ppm and this has resulted in a number of theories as These results do not rule out KCo(Au(CN)2)3 as the main to how they work, but only one theory withstands exam- brightening agent but they strongly indicate that these elements ination. In this theory the semi-metals are regarded as are also present in other forms, for instance some workers have being adsorbed evenly on the cathode surface where they identified elemental cobalt, CoOOH and cobalt cyanide in acid catalytically enhance the nucleation of the gold. In the ab- gold deposits (23,24). sence of these brighteners the gold nucleates at growth Although the formation of KCo(Au(CN),)3 is significant in points such as crystal dislocations on the cathode, but the that it explains many characteristics of the acid gold process, it catalytic effect of the semi-metal encourages more even may well be that this substance is only an important inter- deposition by creating more growth points (17). The mediate in the deposition process. It may be that other reac- effect can be termed catalytic because the charged tions contribute to the brightening process, namely the in- species, for example T1;, is available for readsorption. clusion of potassium, the reduction and deposition of cobalt, The mechanism is even more efficient for sulphite golds. and the formation of complex cyanides and polymers.
78 GoldBull.,1986, 19, (3) Operating Parameters of Gold(I) Cyanide Baths pH pH influences the bath in a number of ways. High pH (— 5.0) allows a higher concentration of free cyanide and therefore cyanocobaltate formation is greater. Also at higher pH the solubility of 1(Co(Au(CN),) 3 increases so that its brightening ability is reduced. As mentioned earlier, chelates are used to control the supply of cobalt to the double layer and chelating strength is pH dependent. Finally, as the bath operates the pH will rise, and to counter this acid gold baths need to be well buffered. pH also affects efficiency in that the lower the pH, the higher the concentration of hydrogen ions; this favours depo- sition of hydrogen instead of gold. Temperature Increasing temperature increases the solubility of A typical composition of an acid gold deposit that is bright, 1(Co(Au(CN),)3 , and this leads to decreasing brightness. Che- has fairly low internal stress and is hard wearing is given in lating strengths are also temperature dependent. The effects of Table I. These figures agree fairly well with those predicted by higher temperature can usually be corrected by increasing the the KCo(Au(CN),)3 theory. gold and cobalt concentrations. The presence in an acid gold plating bath of a strong ligand for cobalt can interfere with the formation of KCo(Au(CN),), Barrel Plating A barrel can act like a semi-permeable membrane by cre- and the brightening action, by removing cobalt ions from the system. Thus, if cobalt is added as the ethylenediamine tetra- ating a separate catholyte within the barrel. Within the anolyte compartment the Co 34 acetate complex, an overall cobalt concentration of about 6 gie " concentration increases, whilst within the catholyte the pH and free cyanide increases. When the bar- may be required to provide sufficient cobalt ions to allow the rel is removed and allowed to drain, the two solutions mix and formation of 1(Co(Au(CN),) 3 . there is a rapid formation of cyanocobaltate. These effects can There is, however, an extremely strong ligand in all acid be countered by maintaining a low pH, increasing the flow of gold cyanide baths, namely cyanide itself. The cyanocobaltate electrolyte through the barrel and by using the largest possible complex Co(CN)3 is one of the most stable complexes known volume of electrolyte. (4), so much so that when complexed to Co(III), cyanide ceases to be toxic. More significantly, when cobalt is complexed with High Speed Plating cyanide it is no longer available to form KCo(Au(CN),),. The High speed acid gold solutions are designed to produce the formation of cyanocobaltate is encouraged by high Co(III) con- same type of deposit as conventional vat plating. In order to do centration and high free cyanide concentration. this it is usual to increase the gold and cobalt content so as to In an acid gold system the free cyanide concentration is very prevent local depletion around the cathode. For the same low, as any free cyanide tends to form undissociated HCN, but reason agitation in, high speed plating becomes critical: jet at higher pH free cyanide concentrations are increased. plating is necessary to produce the required turbulence for Therefore it is important, when baths are analysed for co- very high current densities. Higher temperatures are needed to balt, that the analysis should differentiate between 'inert' and reinforce the effect of turbulence. `active' cobalt. Although the effect is not as strong when iron or The main characteristics of high speed plating are the. high nickel are used as brightening agents, it is nevertheless still anodic and cathodic current densities. Various effects result operative in baths containing these brighteners. from this: the pH rises faster; more Co 3+ is produced; conver- sion to cyanocobaltate is faster; and organic chelates tend to be As yet there is insufficient evidence to say conclusively what anodically oxidised. As far as brightness alone is concerned, the state of the as-deposited cobalt is, but all observers are now semi-metals are better additives at very high current densities. agreed that there is non-metallic material present and that Recently, a number of acid gold processes have been devel- some, if not all, the cobalt is present in the form of a cyanide oped which make use of organic additives to increase the cur- complex. However, the ICCo(Au(CN),), theory does provide an rent density range of baths for deposition of cobalt and nickel explanation of the observed behaviour of cobalt-brightened brightened acid golds. This enables high speed plating to be acid gold baths. done at lower concentrations than was previously possible.
Golc1Bull., 1986, 19, (3) 79 Efficiency Physical Properties of Deposits By efficiency we mean that proportion of the applied cur- The physical properties of gold deposits will now be con- rent, usually expressed as a percentage, that results in the depo- sidered in the light of the above. sition of gold. The remaining current is responsible for the reduction of other species. Hardness The reaction below is of little importance in terms of cur- The hardness of a gold deposit is due to its structure or the rent consumption but the formation and adsorption of impurities in the deposit or both. With acid cyanide deposits KCo(Au(CN),), increases the polarisation and thus encourages there is an almost direct relationship between hardness and the deposition of hydrogen: base metal content, and it is possible to vary the hardness of a deposit by varying the chelate concentration in the electrolyte. Co" + e Co" With organically brightened golds the hardness increases with increasing brightener concentration, the causes of the in- Any parameters that encourage the formation of creased hardness being higher impurity level and greater grain KCo(Au(CN),),, will therefore decrease the efficiency, for refining. The hardness of sulphite golds can be controlled by example low pH, high free cobalt concentration, low tempera- the amount of free brightener, usually arsenic, in the electrolyte ture_ but in this case the hardness is not due to arsenic in the deposit The use of the correct chelating agents will control the avail- but to grain refining. It has now been realised that the re- lationship between hardness and wear resistance is not simple ability of cobalt ions and thus the formation of KCo(Au(CN) 2)3 so that the best compromise between brightness and efficiency and straightforward, so that hardness is not regarded as being can be achieved. as important as it used to be.
Impurities Wear Resistance The effect of impurities can be exemplified by considering Wear resistance is not a definite physical property but lead, which can act as a semi-metal brightener, but which in a depends on very complex interactions of numerous factors. cobalt acid gold bath inhibits the formation of KCo(Au(CN),),. Hence, when comparing wear resistance it is important that the In the light of what has been discussed above about the catalytic same experimental technique is used. Bright deposits have bet- effects produced in semi-metal brightening, it will be seen that ter wear resistance than dull deposits, acid gold bath deposits lead could be expected to interfere with the formation of the are better than organically brightened deposits, and these in brightener and cause depolarisation and an increase in ef- turn have better wear properties than semi-metal brightened ficiency Most semi-metals interfere in this way, but Group 4 cyanide or sulphite golds. The cobalt brightened acid golds contaminants are more acceptable, at least in fairly low concen- have a fine grained structure with a polymer framework which tration. gives a tough deposit, whilst arsenic brightened sulphite de- posits are very fine grained and almost amorphous; their fine Agitation grained structures being easily deformed. Organically bright- Compared with most base metal plating baths, precious ened golds show acceptable wear resistance against them- metal baths have a low metal concentration in order to reduce selves, but against the tough resilient deposits of an acid gold inventory costs. bath they are unacceptable. Nearly all the current in such baths is carried by the con- More recent work (20) has indicated that the level of potass- ducting salt, which is in relatively high concentration. The ium has an important influence on the sliding wear resistance Au(CN)2 ion is not electrostatically attracted to the cathode and of acid gold deposits. However, as the atomic ratios of K:C and the supply of gold to the Helmholtz double layer is thus dif- C:N in such gold deposits are fairly constant, it could be that the fusion dependent. Agitation is therefore important as a means potassium level is only an indication of other features in the of increasing the supply of both gold and brightener to the deposits which are responsible for their improved wear resist- cathode. This has both advantages and disadvantages in that ance. high agitation can increase efficiency, but it also makes distri- bution worse by increasing the efficiency relatively more at Porosity high current densities than at low current densities. If the pre-treatment of the substrate is effective, the fine Modern selective plating techniques have reduced the prob- grained deposits from a semi-metal brightened deposit show lems of metal distribution on the substrate and one of the other less porosity than acid gold deposits. Unfortunately the deposit benefits of the new current ,density extending additives is that structure factors that reduce porosity tend to preclude other they also improve distribution. desirable properties. 80 Gold Bull., 1986, 19, (3) The study of the formation of pores in acid gold deposits has made the formulation of Iow-porosity acid golds possible, but pretreatment still remains the most important factor in control- ling porosity.
Stress There are a number of theories that attempt to explain stress in electrodeposits; none of them are entirely satisfactory and only two will be considered. The excess energy theory (23) states that an ion must pass an energy barrier in order to leave its ligand field and become an atom in a solid lattice. This additional energy barrier is respon- Density sible for polarisation. Once over this energy barrier, the atom Table II shows the densities of acid gold deposits. The figure will have excess energy which is converted into heat so that for pure gold deposit is included. each freshly deposited layer is hotter than the rest of the solid. The much lower density of cobalt and nickel brightened On cooling this gives rise to tensile stress. The relatively high acid gold deposits cannot be explained on the simple assump- stress in acid golds and the low stress in sulphite golds is ex- tion that the deposit contains 0.2 per cent cobalt or nickel. For plained in this way. On this basis, however organically the density to have fallen so much, the structure of the gold de- brightened gold should be high in tensile stress and this is not posit must have been expanded by the inclusion of relatively necessarily so. light weight material. The dislocation theory (24,25) states that impurities cause It has been indicated above that in acid gold deposits up to extra dislocations in the metal lattice, which cause stress in acid 10 per cent of the atoms are not gold but are much lighter el- golds and sulphite golds. In terms of this theory certain ements. This explains their lower density. organics should give rise to compressive stress because they disguise some of the surface vacancies which lead to dislo- Conclusion cations. The early acid gold processes were developed empirically It is known from empirical observations that a deposit con- and it is only in the past few years that the relationship between taining more than 0.3 per cent w/w Co or Ni will probably be the composition and operating parameters of the electrolyte, too highly stressed, and this would fit in with the second theory. and the composition and performance of the deposits have Stress is probably generated in more than one way and it is begun to be understood. If significant advances are to be unlikely, therefore, that one theory will ever supply all the achieved, a more rigorous theoretical basis is required as a answers. guide in exploring avenues of future development. 0
Acknowledgement This paper incorporates contents from an earlier publication (3) by the author. They are reproduced with the permission of the Institute of Metal Finishing.
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Gold Bulk 1986, 19, (3) 81